1. Field of Invention
The field of this invention is related to track-and-hold or sample-and-hold (S&H) circuits. The two terms are normally used interchangably with the slight distinction that there are indeed some sample-and-hold designs that are not capable of tracking a voltage over a period of time, but only of sampling the voltage at a given time and then holding the voltage. For instance, most sampling oscilloscopes use a diode bridge sampler, which because of design constraints needed to obtain high input bandwidth with minimum power, is only capable of sampling and holding as opposed to tracking, sampling and holding. This nomenclature S&H, will be used throughout this patent specification.
The two most important specifications for a high performance S&H are resolution and bandwidth. Since as much as 90% of the commercial applications for S&H are for A-D converters the most relevant way to specify S&H's is in relation to their suitability for a given A-D converter specification. Thus, the resolution is usually specified in number of bits and the maximum sample frequency at which that bit-wise resolution can be guaranteed.
It is generally recognized that for resolutions above 10 or 11 bits, which correspond to a fractional resolution of 1 part in 1024 to 1 part in 2048, it becomes necessary to employ a high accuracy feedback amplifier. Simpler configurations that rely on only switches, capacitors and open loop amplifiers are generally not qualified for 12 bit or higher accuracy requirements because 1 part in 4096 linearity cannot be sustained without feedback. This invention is specifically concerned with configurations that employ feedback in both the tracking/sampling mode as well as in the hold mode.
Specifically, S&H configurations will be described that employ multiplexed feedback amplifier configurations with at least two sets of potentially active differential, dual differential or single-ended inputs and one or more sets of differential or single-ended outputs. Sarkissian 1972 U.S. Pat. No. 3,638,131, teaches how to use series diodes to isolate current mode multiplexed differential amplifiers, Series switches on the inputs is another well-known method of multiplexing. Figures of merit for multiplexed feedback amplifiers include cross-talk or input to output isolation, gain error, offset, etc. As will become clear, all these characteristics of a multiplexed feedback amplifier will determine the corresponding specifications for the S&H that incorporates the multiplexed amplifier.
2. Background of the Invention
Since S&H's are mainly included in a given A-D converter design, due to accurate requirements, it is useful to consider broad groupings of S&H methods based on accuracy. The simplest S&H is simply a series switch charging a single capacitor. The primary disadvantage is lack of isolation in the hold mode. Such first order designs require large hold capacitors for accuracy which reduces the bandwidth of the switch series resistance charging such a large capacitor. Also, the switch usually feeds through some charge in the transition from on to off (i.e., tracking to holding) and the switching charge feedthrough as well as the time at which the sampling occurs are generally not constant with input signal level. These variable effects introduce systematic harmonic distortion that is signal level dependent. Also, slew rate dependent variations of the effective sample time can occur depending on the method of actuating the on to off transition of the series switch.
It is an object of the present invention to reduce these sources of systematic harmonic distortion by providing configurations that employ a ground referenced series switching configuration. In this way, input signal level variations are never experienced by the switch, only slew dependent dynamic current loading is experienced, which is minimized if the switch transistors are properly sized in relation to the dynamic current variation to maintain a high effective F.sub.T. It is a further object of this invention to provide differential cancellation of the switching transition charge feedthrough. This allows for the use of a larger switch/capacitor ratio which increases available bandwidth and/or reduces IC die size and power.
DC offset errors are also very important for A-D converter applications and multiplexed amplifiers have more sources of errors than do ordinary op-amps. It is a further object of this invention to provide S&H configurations that can effectively reduce the DC errors by employing an auto-zero capability. If the auto-zero cycle capability is invoked during each sample/hold cycle then an added advantage is that low frequency noise (1/f), up to about 1/3 of the sampling frequency, can be effectively eliminated.
It is generally recognized that differential signal coupling is advantageous for integrated circuit A-D converter applications. Ground and power supply noise are transformed into common mode noise sources that leave the differential signals unperturbed. However, most signal sources of interest are usually supplied in single-ended form. Transformer coupling is not preferred because of expense and lack of D-C coupling. There are indeed feedback amplifier configurations that can accomplish a single-ended to diferential translation and it is a further object of this invention to provide complete S&H functions that provide a single-ended input signal capability with a differential output signal capability.
Charge-coupled devices (CCD's) have unique signal handling capabilities that require more complex S&H configurations. Specifically, CCD's have a multi-phase clock whereby there is a reference signal output phase and signal output phase. The real signal that is desired for the highest SNR performance is the difference between the reference and signal. The signal that represents this difference is called a double correlated sample and such systems of sampling are generically referred to as double correlated sampling. The ability to output such a signal is not intrinsic to CCD's themselves, but must be added with appropriate S&H functions. Usually, two separate S&H's are required as well as a method of subtracting the two values. It is a further object of the present invention to provide S&H means that directly provide double correlated sampling capability using only one multiplexed amplifier and that, further, this be made possible with zero offset correction capability.
The general field of A-D converter designs that can usefully employ an S&H include successive approximation, pipelined successive approximation and recirculating. The minimum complexity recirculating A-D converter requires several S&H functions, a gain of two amplifier capability, a 1 bit D-A offsetting capability and a 1 bit flash A-D, as well as the necessary control signals and digital logic. Recirculating A-D converters are very hard to manufacture because of the difficulty in maintaining an exact gain of two amplifier with zero offset as well as of supplying the D-A offsetting function with absolute 16 bit linearity. It is a further object of the present invention to provide an S&H function that has two distinct signal input capabilities, each of which can be selectively held on the output, that has a high accuracy gain of two transfer function, that uses double correlated sampling type capabilities to selectively actuate a 1 bit D-A offsetting capability and that is also capable of zero offset correction for each of the input signal capablities, all to a high accuracy capable of supporting 16 bit recirculating A-D converter configurations.